When a young boy has sexual fantasies about being a girl and sitting on a 'sub-woofer' that is playing Dubstep music causing them to get extremely aroused by the vibrations of the bass which eventually send them into sexual 'ecstasy'.

The dubstep song playing on John's computer made him go into 'substasy'.

Clair: Hey Tom, what did you think of that Dubstep song i sent you?
Tom: Ohh girl, it felt like 'substasy'.
Clair: ew